In a radial tire including a belt layer that is disposed outside of a carcass and a tread portion being reinforced to exhibit hoop effects, the restraining force of the belt layer will be decreased at an outer end portion thereof so that lifting is apt to be caused at a shoulder portion of the tire through centrifugal force or other factors when performing high speed running. The durability consequently tends to be decreased such that, for instance, peeling and damages are caused originating at the outer end of the belt layer.
To suppress the lifting and increase the high speed running durability performance, conventionally, a band layer around which band cords made of nylon or similar are spirally wound in a tire circumferential direction is provided outside of the belt layer so as to complement restraining force at the outer end portion of the belt layer. It is known that a tire reinforced by the band layer exhibits superior high speed durability performances in high speed durability tests (for instance, standard tests such as ECE 30 etc.) of so-called step speed method in which speed up is performed by a ratio of 10 km/h per every 20 minutes.
Through various experiments that have been performed by the inventors, it has been found that no sufficiently satisfactory effects in durability have been achieved by the above tire when continuous high speed running was made under the condition close to actual usage such as in the case where it was made to run at high speed for a long period of time. This is assumed to be due to the fact that the buttress portion outside in the tire axial direction is more largely affected rather than the belt layer, wherein the buttress portion is gradually raised by the high speed running lasting for a long time so as to cause lifting of the shoulder portion.
The present invention thus aims to provide a pneumatic radial tire in which movements of the buttress portion and lifting of the shoulder portion are effectively restricted, and high speed durability is improved not only when a step speed method is employed, but also when continuous high speed running is conducted under the condition close to actual usage. For such purpose, the present invention basically provides a structure of a pneumatic radial tire in which a cord-reinforced layer, the radially outer end thereof being secured between a carcass and a belt layer and the radially inner end thereof being secured between a ply main portion and a ply turnup portion of the carcass, is provided at the buttress portion.